Frost Hardening Research Articles

Autumn cold acclimation of shoots does not explain the northern distribution limit of three southern exotic tree species in Finland

Frost hardiness (FH) is one of the key traits that limits the distribution of tree species in the north. Different species and ecotypes respond differently to the drivers of frost hardening and may therefore have different survival capacities, especially in their northern distribution range. Several southern tree species such as pedunculate oak (Quercus robur L.), Norway maple (Acer platanoides L.), and mountain elm (Ulmus glabra Hudson) have their natural northern distribution limit in Southern Finland. Here, we examined whether insufficient frost hardening in the autumn limits the northwards distribution of these three species. To that end, we monitored phenological events and frost hardening in the autumn in these exotic species in Central Finland, further north from the natural distribution range of the three exotic species, and for the sake of comparison, in the native silver birch (Betula pendula Roth.). In all species, a timely growth cessation was observed. The water content of the stem decreased, and its dry mass content increased rapidly in August before the onset of frost hardening, after which no major changes were observed in these two traits. In the early phase of hardening, there were differences in leaf colouring between the four species. The values of the electrical impedance parameters of the stem changed during frost hardening, with a clear turning point after the first night frosts at the end of September. Depending on the species and hardening phase, the high temperature exotherm (HTE) of the stem was observed between -3 °C and -10 °C, but HTE did not indicate differences between the species in frost hardening. In all four species, frost hardening started at the beginning of September and proceeded first at equal rates, but the hardening was more rapid later in birch than in the other species. The damaging temperature range assessed by the controlled freezing tests became wider with increasing FH. The occurrence rate of the low temperature exotherm (LTE) varied between the species, and the LTE was observed at around -35 °C in the last sampling at the beginning of December (no LTE was observed in birch). We conclude that the hardening of all four species took place in a timely manner so that in each, the frost hardiness was sufficient to survive the autumn frosts at the experimental site in Central Finland. Further studies are needed to examine whether the exotic three species also tolerate temporarily occurring extreme winter conditions such as temperatures below -35 °C, and the high short-term temperature variations in winter and especially in spring.

Cold acclimation and deacclimation in wild blueberry: Direct and indirect influence of environmental factors and non-structural carbohydrates

Through the annual cycle of plant growth and dormancy, the winter season leads to profound metabolic changes allowing plants to undergo cold acclimation. In boreal environments, winter conditions are changing rapidly and are likely to cause damage to commercial wild lowbush blueberry. In this study, we addressed the level of frost hardiness and determined the role of environmental factors and nonstructural carbohydrates (NSCs) on frost hardiness. From autumn to spring, stem sections of Vaccinium angustifolium and Vaccinium myrtilloides were harvested each month in a commercial blueberry field to assess the relative electrolyte leakage and calculate the temperature at which 50% of the cells are lysed [LT50 ( °C)], used as frost hardiness index. Stems were also collected to assess soluble carbohydrates and starch. Correlations, principal component analysis (PCA) and structural equation modeling (SEM) were used to determine how environmental factors and NSCs directly or indirectly influence the frost hardiness index. Frost hardiness reached its lowest level in December and January with LT50 dropping below -60 °C. Seasonality of frost hardening was closely linked to photoperiod and temperature, generating clock-wise hysteretic loops that divide frost hardening into acclimation, from September to January, and deacclimation, from January to the end of May. Environmental factors such as photoperiod and temperature were more important in determining the level of frost hardiness during acclimation, with either direct or indirect effect through an influence on starch degradation, increasing soluble carbohydrate content. During deacclimation, soluble carbohydrates, especially raffinose, further induced a stronger direct regulation of frost hardiness. Direct biological regulation through raffinose defined the level of frost hardiness during deacclimation. However, the negative influence of temperature on raffinose concentration could increase vulnerability to winter warming events.

The Roots of Plant Frost Hardiness and Tolerance.

Frost stress severely affects agriculture and agroforestry worldwide. Although many studies about frost hardening and resistance have been published, most of them focused on the aboveground organs and only a minority specifically targets the roots. However, roots and aboveground tissues have different physiologies and stress response mechanisms. Climate models predict an increase in the magnitude and frequency of late-frost events, which, together with an observed loss of soil insulation, will greatly decrease plant primary production due to damage at the root level. Molecular and metabolic responses inducing root cold hardiness are complex. They involve a variety of processes related to modifications in cell wall composition, maintenance of the cellular homeostasis and the synthesis of primary and secondary metabolites. After a summary of the current climatic models, this review details the specificity of freezing stress at the root level and explores the strategies roots developed to cope with freezing stress. We then describe the level to which roots can be frost hardy, depending on their age, size category and species. After that, we compare the environmental signals inducing cold acclimation and frost hardening in the roots and aboveground organs. Subsequently, we discuss how roots sense cold at a cellular level and briefly describe the following signal transduction pathway, which leads to molecular and metabolic responses associated with frost hardening. Finally, the current options available to increase root frost tolerance are explored and promising lines of future research are discussed.

Evaluating multiple bioclimatic risks using Bayesian belief network to support urban tree management under climate change

Understanding the vulnerability of trees affected by climate change is a key requirement for identifying management priorities and suggesting suitable urban tree species. To measure such vulnerability under changing climate conditions, indicators of bioclimatic characteristics should be identified and evaluated using past and current geographic growth ranges. However, although climate events often occur simultaneously (e.g., frost and drought), and management issues in this regard need to be clarified, it is challenging to consider multiple risks in a climate change vulnerability assessment. Therefore, we applied a Bayesian belief network (BBN) to interlink the bioclimatic requirements of species and seasonal climate risk of the study site to comprehensively assess the multiple risks. In particular, we integrated expert knowledge and supporting evidences from relevant studies to construct the BBN. The developed BBN revealed vulnerability to frost considering occurrences of cascading and co-occurring climatic risks such as warmer winters and droughts throughout the phenological cycle. As a case study, two tree species, Zelkova serrata and Camellia japonica from Seoul, Republic of Korea, were evaluated. Among the climatic risks considered, the BBN revealed that shortened frost hardening and the occurrence of spring frost right after an extraordinarily warm winter would mainly affect vulnerability to frost of the two species. In particular, C. japonica had high vulnerability due to its high susceptibility to coldness, though growing temperature will be perfectly satisfied under climate change. Generally, this study provides insights to consider multiple bioclimatic risks for guiding urban tree management under climate change.

Biophysical changes in the roots of Scots pine seedlings during cold acclimation and after frost damage

Root system health is a key factor for seedling quality and it is a prerequisite for the proper growth of seedlings after out-planting. If the seedlings are moved to freezer storage too early in the season, roots can be damaged and thereby the quality of seedlings declines. We aimed to develop a novel method to assess an appropriate time for moving the seedlings to freezer storage in the autumn and to detect possible root damage during overwintering. One-year-old containerized Scots pine seedlings were measured before, during and after freezer storage, and after a series of frost exposure tests. The electrical impedance spectra (from 4 kHz to 200 kHz) of roots changed during cold acclimatization. The impedance loss factor (δ) of roots at 50 kHz frequency decreased when the threshold of frost tolerance was exceeded in the freezing tests. Root hydraulic conductance (Kr) increased before and during the freezer storage. In the initial phase of cold acclimatization, Kr increased considerably after exposure to frost temperatures, indicating damage, but that effect disappeared with frost hardening. In regrowth tests, the largest number of new root tips was observed after exposure to −12 °C in the freezing tests just before and after freezer storage, and after exposure to −30 °C during the freezer storage. During the freezer storage, shoot growth declined at higher exposure temperatures than root tip formation, suggesting that roots and specifically root tips would not be the primary reason for declined shoot growth. We conclude that the biophysical measurements of roots are useful for assessing the condition of the root system for overwintering applications in tree seedling nurseries, and that roots tolerated lower temperatures than previously thought.

Does winter desiccation account for seasonal increases in supercooling capacity of Norway spruce bud primordia?

Bud primordia of Picea abies (L.) H. Karst. remain ice free at subzero temperatures by supercooling. Once ice forms inside the primordium, it is immediately injured. Supercooling capacity increases seasonally from ~−5 °C to as much as −50 °C by currently unknown mechanisms. Among other prerequisites, dehydration of tissues over the winter months has been considered to play a key role in freezing tolerance. In this regard, the water content of bud primordia may be crucial, especially in reference to supercooling. In order to assess the role of dehydration in supercooling capacity, seasonal changes in supercooling capacity and the water potential of bud primordia of Picea abies (L.) H. Karst were measured at two sites that differed by 1298 m in elevation, after artificial frost hardening and dehardening treatments and after controlled bench drying. The extent of supercooling of bud primordia varied from −7 °C in summer to −24.6 °C in winter, a difference of 17.6 –19.3 K. Total actual water potential (Ψtact) of bud primordia was −2 MPa in summer and decreased to a mean of −3.8 MPa in midwinter. The decline involved dehydration, and to a lesser extent, osmoregulation. At decreased Ψtact values (<3.0 MPa), supercooling capacity significantly increased <−19.5 °C, however, the correlation between actual water potential and supercooling capacity was poor. Frost-hardening treatments increased the supercooling capacity of bud primordia (−0.6 K day−1) and lowered Ψtact (−0.2 MPa day−1). Frost-dehardening treatments reduced supercooling capacity (+1.1 K day−1), and at the same time, increased Ψtact (+0.3 MPa day−1). In contrast, artificial drying of bud primordia in the range observed seasonally (−2.0 MPa) had no effect on supercooling capacity. These results suggest that there is no causal relationship between desiccation and the supercooling capacity of bud primordia in P. abies, but rather it involves other compounds within the cells of the bud primordium that reduce the water potential.

Frost hardening and dehardening potential in temperate trees from winter to budburst.

We investigated how deciduous trees can adjust their freezing resistance in response to temperature during the progress of the ecodormancy phase, from midwinter to budburst. We regularly sampled twigs of four different temperate deciduous tree species from January to the leaf-out date. Using computer-controlled freezers and climate chambers, the freezing resistance of buds was measured directly after sampling and also after the application of artificial hardening and dehardening treatments, simulating cold and warm spells. The thermal time to budburst in forcing conditions (c. 20°C) was also quantified at each sampling as a proxy for dormancy depth. Earlier flushing species showed higher freezing resistance than late flushing species at either similar bud development stage or similar dormancy depth. Overall, freezing resistance and its hardening and dehardening potential dramatically decreased during the progress of ecodormancy and became almost nil during budburst. Our results suggest that extreme cold events in winter are not critical for trees, as freezing resistance can be largely enhanced during this period. By contrast, the timing of budburst is a critical component of tree fitness. Our results provide quantitative values of the freezing resistance dynamics during ecodormancy, particularly valuable in process-based species distribution models.

Effects of Scots pine paternal genotypes of two contiguous seed orchards on the budset and frost hardening of first-year progeny

Effects of Scots pine paternal genotypes of two contiguous seed orchards on the budset and frost hardening of first-year progeny

Variability of phenology and fluxes of water and carbon with observed and simulated soil moisture in the Ent Terrestrial Biosphere Model (Ent TBM version 1.0.1.0.0)

Abstract. The Ent Terrestrial Biosphere Model (Ent TBM) is a mixed-canopy dynamic global vegetation model developed specifically for coupling with land surface hydrology and general circulation models (GCMs). This study describes the leaf phenology submodel implemented in the Ent TBM version 1.0.1.0.0 coupled to the carbon allocation scheme of the Ecosystem Demography (ED) model. The phenology submodel adopts a combination of responses to temperature (growing degree days and frost hardening), soil moisture (linearity of stress with relative saturation) and radiation (light length). Growth of leaves, sapwood, fine roots, stem wood and coarse roots is updated on a daily basis. We evaluate the performance in reproducing observed leaf seasonal growth as well as water and carbon fluxes for four plant functional types at five Fluxnet sites, with both observed and prognostic hydrology, and observed and prognostic seasonal leaf area index. The phenology submodel is able to capture the timing and magnitude of leaf-out and senescence for temperate broadleaf deciduous forest (Harvard Forest and Morgan–Monroe State Forest, US), C3 annual grassland (Vaira Ranch, US) and California oak savanna (Tonzi Ranch, US). For evergreen needleleaf forest (Hyytiäla, Finland), the phenology submodel captures the effect of frost hardening of photosynthetic capacity on seasonal fluxes and leaf area. We address the importance of customizing parameter sets of vegetation soil moisture stress response to the particular land surface hydrology scheme. We identify model deficiencies that reveal important dynamics and parameter needs.

Frost hardening of Scots pine seedlings in relation to the climatic year-to-year variation in air temperature

The study addressed the effect of year-to-year variation in air temperature on the early needle frost hardening of first-year Scots pine (Pinus sylvestris L.) seedlings in autumn. To this end a novel method, combining biometeorological and plant ecophysiologal research, was introduced. In the biometeorological part, climatic year-to-year variation in air temperature during August and September was examined by analysing a 51-year set of daily air temperature data from central Finland. The cumulative occurrence of low air temperatures in August and September was quantified by calculating a cold sum for that period each year, and the climatic year-to-year variation of the cold sum accumulation was subsequently determined. Similar cold sum calculations were carried out for different air temperature treatments in hypothetical growth chamber experiments. By comparing the results of these two sets of calculations, experimental designs were defined for air temperature treatments covering the climatic year-to-year variation of cold sum accumulation in August and September. In the ecophysiological part of the study, the effects of low air temperatures on the early needle frost hardening of Scots pine seedlings were studied experimentally over two years in central Finland, using the air temperature treatments defined in the first part of the study. The hardening treatments were implemented in growth chambers under conditions simulating natural autumn conditions: gradually increasing night length combined with decreasing and diurnally fluctuating air temperature. Out of the eight air temperature treatments applied, only one had a clear effect of accelerating the hardening. Comparisons of the cold sum accumulations in the experimental treatments with those in natural conditions suggest that low air temperatures do not accelerate the early frost hardening of Scots pine seedlings in most years; further experimental studies are needed in order to determine the frequency of the years when they do. The results of the cold sum comparisons in the present study will help to identify the experimental designs needed in forthcoming studies.

Use of the delayed luminescence test for evaluation of changes in frost-resistance of winter wheat

The influence of frost hardening conditions on the frost-resistance of three winter wheat cultivars ('Mironowska 808', 'Grana' and 'Maris Huntsman') was studied. Frost resistance was measured periodically during the autumn and winter season by the delayed luminescence method. Parallelly an electrical conductivity test was performed for comparison. It was found that the temperature which delayed luminescence reaches its maximum (t&lt;sub&gt;N&lt;/sub&gt;) correlates with (t&lt;sub&gt;50&lt;/sub&gt;) obtained by the conductometric method. The differences in response of particular cultivars to frost and hardening conditions are described.

Frost as a limiting factor for recruitment and establishment of early development stages in an alpine glacier foreland?

AbstractQuestionsHow frost resistant are the early development stages (seeds, seedlings, plantlets and juveniles) of alpine plant species? Do summer frosts impair establishment of plant species typical of different successional stages on a central alpine glacier foreland?LocationRotmoos glacier foreland, Austrian Central Alps (Obergurgl, Tyrol, Austria).MethodsSeeds of 12 species typical of different successional stages were collected in the glacier foreland and either sown directly in the field or in a growth chamber (25/10 °C, 16/8 h) and grown to the investigated development stages. Frost resistance of the early development and adult stages was determined by exposing them to a set of freezing temperatures and assessing viability with the tetrazolium test (LT 50, i.e. 50% of samples being lethally frost damaged).ResultsDry seeds had the highest frost resistance (LT 50: −19 °C), followed by wet seeds after imbibition (LT 50: −8 °C). With the onset of germination, frost resistance decreased rapidly. While germinated seeds tolerated a mean of −3.2 °C, seedlings and juveniles were less frost resistant (LT 50: −2.5 °C). Along the primary succession, seedlings of pioneer species were significantly less frost resistant than early‐ and late‐successional species. However, field grown seedlings, mainly of pioneer species, showed higher frost resistance (mean: −5 °C) than the growth chamber seedlings (mean: −3 °C), indicating that frost hardening (transition from a lower to a higher level of frost resistance) is already possible during these early stages of development.ConclusionsThe low frost resistance during and after germination may not suffice to survive summer frosts and may at least in certain years explain the high seedling mortality rates recognized in the glacier foreland.

Decreased frost hardiness of Vaccinium vitis‐idaea in reponse to UV‐A radiation

The aim of this study was to investigate plant frost hardiness responses to ultraviolet (UV) radiation, since the few results reported are largely contradictory. It was hypothesized that functional adaptation of life forms could explain these contradictions. Dwarf shrubs and tree seedlings, representing both evergreen and deciduous forms, were tested (Vaccinium vitis-idaea, Vaccinium myrtillus, Pinus sylvestris, Betula pubescens and its red form f. rubra). The research was performed in Sodankylä, Northern Finland (67°N), with enhanced UV-B- and UV-A-radiation treatments between 2002 and 2009. Plant frost hardiness was determined using the freeze-induced electrolyte leakage method in early autumn, during the onset of the frost hardening process. Additional physiological variables (malondialdehyde, glutathione, total phenols, C and N contents) were analyzed in V. vitis-idaea to explain the possible responses. These variables did not respond significantly to UV-radiation treatments, but explained the frost hardiness well (r² = 0.678). The main finding was that frost hardiness decreased in the evergreen shrub V. vitis-idaea, particularly with enhanced UV-A radiation. No significant responses were observed with the other plants. Therefore, this study does not support the idea that enhanced UV radiation could increase plant frost hardiness.

COMPARISON OF THE FROST RESISTANCE OF BARLEY (HORDEUM VULGARE L.) LANDRACES OF UPLAND ETHIOPIA USING ELECTROLYTE-LEAKAGE AND CHLOROPHYLL FLUORESCENCE

Barley is the most important crop in the highlands of Ethiopia at altitudes above 2600 m, where its productivity is limited by cold stress. We studied 25 Ethiopian barley landraces in order to identify cold tolerant types and to describe characteristics and acclimation potentials of these landraces to cold stress. Barley plants were grown from seeds in a temperature-controlled greenhouse (20/15°C) for six weeks prior to investigation. Membrane leakage, assayed by electric conductivity, was investigated before and after the plants had been subjected to freeze-thaw cycles at different sub-freezing temperatures down to -10°C. The critical temperature (CT 50 ), at which the frost-induced damage was 50% of the maximal injury, ranged for most landraces between -5.6 and -8.6°C. Maximum difference in sensitivity to frost among the various races was observed at -5.0°C. Neither the altitude of the habitat, from where the seeds had been collected, nor the maturation time of the landraces were correlated with the degree of frost tolerance of the landraces. The influence of the growth conditions on the capability of frost hardening of cold-tolerant and sensitive landraces was investigated using chlorophyll a fluorescence. Barley seedlings were grown under three different regimes: In a temperature controlled greenhouse (20/15°C, day/night), under simulated tropical alpine conditions (tropical-alpine greenhouse, 16/0°C, 12 h light period), and in a phytotrone at constant 2°C day and night. Effective quantum yield of photosynthesis was measured at several temperatures in the course of freeze-thaw cycles (+20 to -10 to +20°C). Whereas growth and the effective photosynthetic quantum yield of the plants in the phytotrone were low, cold hardening was effective as shown by the recovery of the photosynthetic quantum yield upon re-warming of the frozen plants. Effective quantum yield of the plants grown in the tropical alpine greenhouse was as high as with the control plants, but only 2 of 6 investigated landraces showed immediate recovery of the photosynthetic quantum yield after freezing. Landraces which showed the highest cold tolerance were found to acclimatize best.

Winter frost resistance of Pinus cembra measured in situ at the alpine timberline as affected by temperature conditions

Winter frost resistance (WFR), midwinter frost hardening and frost dehardening potential of Pinus cembra L. were determined in situ by means of a novel low-temperature freezing system at the alpine timberline ecotone (1950 m a.s.l., Mt Patscherkofel, Innsbruck, Austria). In situ liquid nitrogen (LN₂)-quenching experiments should check whether maximum WFR of P. cembra belonging to the frost hardiest conifer group, being classified in US Department of Agriculture climatic zone 1, suffices to survive dipping into LN₂ (-196 °C). Viability was assessed in a field re-growth test. Maximum in situ WFR (LT₅₀) of leaves was <- 75 °C and that of buds was less (-70.3 °C), matching the lowest water contents. In midwinter, in situ freezing exotherms of leaves, buds and the xylem were often not detectable. Ice formed in the xylem at a mean of -2.8 °C and in leaves at -3.3 °C. In situ WFR of P. cembra was higher than that obtained on detached twigs, as reported earlier. In situ LN₂-quenching experiments were lethal in all cases even when twigs of P. cembra were exposed to an in situ frost hardening treatment (12 days at -20 °C followed by 3 days at -50 °C) to induce maximum WFR. Temperature treatments applied in the field significantly affected the actual WFR. In January a frost hardening treatment (21 days at -20 °C) led to a significant increase of WFR (buds: -62 °C to <- 70 °C; leaves: -59.6 °C to -65.2 °C), showing that P. cembra was not at its specific maximum WFR. In contrast, simulated warm spells in late winter led to premature frost dehardening (buds: -32.6 °C to -10.2 °C; leaves: -32.7 to -16.4 °C) followed by significantly earlier bud swelling and burst in late winter. Strikingly, both temperature treatments, either increased air temperature (+10.1 °C) or increased soil temperature (+6.5 °C), were similarly effective. This high readiness to frost harden and deharden in winter in the field must be considered to be of great significance for future winter survival of P. cembra. Determination of WFR in field re-growth tests appears to be a valuable tool for critically judging estimates of WFR obtained on detached twigs in an ecological context.

Growth and frost hardening of European aspen and backcross hybrid aspen as influenced by water and nitrogen

• Introduction The interactive effects of water and nitrogen (N) on frost hardiness are not well known in broad-leaved trees. Furthermore, new environmental conditions may favour naturally generated hybrids between native and introduced tree species over native species.

European aspen and hybrid aspen under changing environment – Leaf traits, growth and litter decomposition

Understanding the responses of species and ecosystems to human-induced global environmental change has become a high research priority. The main aim of this thesis was to investigate how certain environmental factors that relate to global change affect European aspen (Populus tremula), a keystone species in boreal forests, and hybrid aspen (P. tremula × P. tremuloides), cultivated in commercial plantations. The main points under consideration were the acclimatization potential of aspen through changes in leaf morphology, as well as effects on growth, leaf litter chemistry and decomposition. The thesis is based on two experiments, in which young aspen ( 8 years) were growing in environments exposed to multiple environmental stress factors (roadside and urban environments). The field studies included litter decomposition experiments. The results show that young aspen, especially the native European aspen, was sensitive to O3 in terms of visible leaf injuries. Elevated O3 resulted in reduced biomass allocation to roots and accelerated leaf senescence, suggesting negative effects on growth in the long term. Water and N availability modified the frost hardening of young aspen: High N supply, especially when combined with drought, postponed the development of frost hardiness, which in turn may predispose trees to early autumn frosts. This effect was more pronounced in European aspen. The field studies showed that mature aspen acclimatized to roadside and urban environments by producing more xeromorphic leaves. Leaf morphology was also observed to vary in response to interannual climatic variation, which further indicates the ability of aspen for phenotypic plasticity. Intraspecific variation was found in several of the traits measured, although intraspecific differences in response to the abiotic factors examined were generally small throughout the studies. However, some differences between clones were found in sensitivity to O3 and the roadside environment. Aspen leaf litter decomposition was retarded in the roadside environment, but only initially. By contrast, decomposition was found to be faster in the urban than the rural environment throughout the study. The higher quality of urban litter (higher in N, lower in lignin and phenolics), as well as higher temperature, N deposition and humus pH at the urban site were factors likely to promote decay. The phenotypic plasticity combined with intraspecific variation found in the studies imply that aspen has potential for withstanding environmental changes, although some global change factors, such as rising O3 levels, may adversely affect its performance. The results also suggest that the multiple environmental changes taking place in urban areas – which correspond closely with the main drivers of global change – can modify ecosystem functioning by promoting litter decomposition, mediated partly by alterations in leaf litter quality.

Critically reduced frost resistance of Picea abies during sprouting could be linked to cytological changes

Frost resistance of sprouting Picea abies shoots is insufficient for survival of naturally occurring late frosts. The cellular changes during sprouting appeared to be responsible for frost damage as frost events that damaged sprouting shoots did not damage older needles and stems. Whilst resting buds showed initial frost damage at -15.0 degrees C, 20 days later, current year's growth was damaged at -5.6 degrees C. The decrease in frost resistance in sprouting shoots of P. abies was accompanied by a significant reduction of the cellular solute concentration, indicated by much less negative Psi(oSAT) values (increase from -2.8 to -1.2 MPa). psi(oSAT) decreased again after the final cell volume was reached and cell wall thickening began. After bud break, ice nucleation temperature increased from -4.7 degrees C to -1.5 degrees C. This increase was probably caused by the loss of bud scales, the onset of expansion growth of the central cylinder and the development of vascular tissue permitting the spread of ice from the stem into the growing needles. The onset of mesophyll cell wall thickening coincided with the lowest frost resistances. Cell wall thickening caused an increase in the modulus of elasticity, epsilon, indicating a decrease in tissue elasticity and after that frost resistance increased again. Metabolic and cytological changes that evidently leave little leeway for frost hardening are responsible for the low frost resistance in current year's growth of P. abies. This low frost resistance will be significant in the future as the risk of frost damage due to earlier bud break is anticipated to even further increase.

Light-induced frost tolerance differs in winter and spring wheat plants

It has been shown that the frost hardening of wheat plants at low temperature under low light conditions is much less effective than under normal light conditions. The aim of the present work was to uncover the biochemical changes responsible for the enhanced freezing tolerance of wheat plants with different levels of frost tolerance when frost hardening takes place in the light. Ten-day-old plants grown at 20/18 degrees C were cold hardened at 5 degrees C for 12d under normal (250micromolm(-2)s(-1)) or low light (20micromolm(-2)s(-1)) conditions. Some of the plants were kept at non-hardening temperature for 12d at elevated light (500micromolm(-2)s(-1)). Changes in the polyamine contents during low temperature hardening showed marked light dependence; however, these changes differed in the spring and winter wheat plants. In the winter wheat Mv Emese, the putrescine, spermidine and cadaverine contents showed a substantial increase at low temperature under normal light conditions, while in the spring wheat Nadro, only the levels of spermidine and spermine increased under these conditions. Low temperature hardening induced a similar level of glutathione S-transferase activity in both Mv Emese and Nadro. However, this induction was more pronounced under normal light than under low light. In the next step, three winter wheat varieties with a high level of frost tolerance and three spring varieties with a low level of frost tolerance were tested for freezing tolerance after being kept at normal, non-hardening temperature at elevated light. The results showed that, although elevated light increased the survival rate of spring varieties as well, even after this hardening process winter varieties had higher frost tolerance than spring varieties. This suggests that there is a correlation between the freezing tolerance induced by low temperature and that induced by light at non-hardening temperature.

Relationship between parameters of electrical impedance spectroscopy and frost hardiness in stems and needles of Pinus bungeana

We studied the frost hardiness (FH) in stems and needles of different Pinus bungeana provenances during frost hardening by means of electrical impedance spectroscopy (EIS) and conventional electrolytic leakage (EL) and compared the regression equations of the two methods in order to optimize the EIS method for studying FH on plants. During frost hardening, EIS and EL were applied to one-year old stems and needles of P. bungeana in an 8-year provenance field trial at the Thirteen Tombs Nursery in Beijing within the provenances of Mangshan of Beijing, Liangdang of Gansu and Xiaoyi of Shanxi provinces, China. A double-DCE model and Model-A were used, respectively, for the EIS analysis of stems and needles that were not exposed to a controlled freezing treatment. After controlled freezing tests, the FH of stems and needles were assessed by EIS and EL. Without controlled freezing tests, the relaxation time (τ1) of stems and the specific intracellular resistance (ri) of stems and needles displayed a statistically significant correlation with FH (R2 = 0.79–0.86); after controlled freezing tests, specific extracellular resistance (re) of the stems and needles, the cell membrane time constant (τm) of needles displayed an even higher correlation with FH (R2 = 0.92–0.94). There were significant relationship between EIS and EL in assessing the FH of stems and needles of P. bungeana, but EIS underestimated FH more than EL did. EIS is one of the more promising methods for assessing FH, especially without employing a controlled freezing test.